Just a quick technique question before I cold crash 2 beers... I've never done this before, and want to be successful on my first attempt.

I've read a few posts on the subject and just want to validate the technique I'm going to use.

I'll remove the airlocks and replace with aluminum foil secured with rubber bands. I'll place the carboys in the freezer and set the temp controller to 40 degrees. I'll leave at that temperature for 18-24 hours.

Opinions vary, but I would leave it for 1-3 (or more) days. There's a lot of thermal mass. It'll take a while just to get the temperature down. For me, 40 is a little high. I go as close to freezing (without freezing) as I can get.

edit: Are you crashing a primary or secondary vessel?

I use to leave an S type airlock on during cold crashing, but now prefer minimizing the chance of exchange of ambient air by using foil and a rubber band.

From what I've read, cold crashing creates a vacuum in the carboy. People have stated in some of the threads on HBT, and on other boards, that they've experienced "suck back"... meaning that the liquid in the airlock is being sucked into the carboy as a result of the vacuum. One suggestion was to remove the airlock and replace it with a piece of sanitized aluminum foil. Sounded reasonable to me.

From what I've read, cold crashing creates a vacuum in the carboy. People have stated in some of the threads on HBT, and on other boards, that they've experienced "suck back"... meaning that the liquid in the airlock is being sucked into the carboy as a result of the vacuum. One suggestion was to remove the airlock and replace it with a piece of sanitized aluminum foil. Sounded reasonable to me.

The possibility of suck-back is why you put StarSan in the airlock. If it gets in the beer, no biggie. Also, the S airlocks, unless overfilled with liquid, aren't prone to that problem.

Agreed...i leave my S type airlock on. I am also a big fan of buckets so it would be hard to remove the airlock and then have to cover the hole...
I also cold crash for a minimum of 2 days...sometimes longer. it wont hurt anything to let it sit...but i alwasy leave it for 2 days since it takes a while to get down in temp. then you want to give it time once it is at temp to drop everything.

The "vacuum" comes from the fact that, as the beer is cooled, the gas in the headspace above the beer "compacts" and more dense. This effect pulls air in from the outside through the airlock.

If the headspace is relatively small, the resulting draw in will also be small. If the airlock is properly filled, chance of getting any of the airlock liquid pulled into the fermenter should be minimal.

On the other hand, if the size fermenting vessel has not been chosen properly and there is a large head space and the airlock is overfilled with liquid, the chances of draw back are greatly increased.

I don't ever remember having liquid sucked into any of my fermenters in nearly 20 years of brewing.

From what I've read, cold crashing creates a vacuum in the carboy. People have stated in some of the threads on HBT, and on other boards, that they've experienced "suck back"... meaning that the liquid in the airlock is being sucked into the carboy as a result of the vacuum. One suggestion was to remove the airlock and replace it with a piece of sanitized aluminum foil. Sounded reasonable to me.

yup it will do that. you can use foil or just let what ever you are using in your airlock. (vodka star san) get sucked in a bit. No big deal there. The problem is when you still have a blow off tube attached you run a risk of sucking in alot of fluid.

on the cold crash its better if you drop your temps slower to keep your yeast from puking up any off flavors. I like to do it over the course of a day and leave it at temp for 24 hrs.

on the cold crash its better if you drop your temps slower to keep your yeast from puking up any off flavors. I like to do it over the course of a day and leave it at temp for 24 hrs.

This sort of slow downward temp adjustment really isn't necessary.

If you put 5 gallons of 68*F beer into a fridge that's set at 35*F, it's going to take some hours for the liquid to cool to 35*F.

Also, the idea that "quickly" dropping the temp of a completed batch (after the yeast have already finished fermenting, have dropped out and gone nighty-night) is, IMO, a brewing myth. That sort of comment gets posted every so often, but I've yet to see anything to back it up. It's likely a conclusion that someone once leaped to based on the known risk of off-flavors being produced when an active fermentation is exposed to sudden, significant fluctuations in temperature.